The level of Na-pump activity has ben implicated in mechanisms which could contribute to human obesity. (Na+ + K+) ATPase activity is reduced in intact red cells and isolated erthrocyte membranes from obese Pima Indian subjects. Fibroblasts, white cells and adipocytes are being examined to determine if there is a widespread tissue loss of sodium pump activity. These cells are also being tested for a response of the Na-pump to hormones which stimulate thermogenesis: Studies of the sodium pump activity red cells from thin and obese families and in diploid fibroblasts will be used to determine possible genetic differences in the sodium pump which relate to obesity.